The invention relates to hyperfiltration membranes (HFM) and to processes for their production.
Membranes are thin flat foils and films which are distinguished by a certain strength and elasticity. They may consist of plastics or natural products and, by virtue of their filter effect, have considerable significance in separation technology. For example, membranes of high polymers are used for industrial separation processes, such as desalination by reverse osmosis, for dialysis, ultrafiltration, hyperfiltration, etc. Hyperfiltration is used in particular for the separation of high molecular weight substances from solutions. Particularly fine-pored membranes and high pressures are used for this purpose.
HFM's of cellulose acetate are most widely used. In addition, membranes of polyamides, fluorine-containing polymers and also polyureas and sulfonated polyarylethers are acquiring increasing interest. Membranes of these materials are anisotropic because they consist of several vertically adjacent layers having different physical-chemical properties. For example, they consist of a thick porous supporting layer and a comparatively thin and compact separation layer. In all these layers, the polymer molecules are arranged in the form of a felt. Chain intervals, chain interval distribution, fixed-charge concentration, polarity and the swelling capacity of the polymer chains in these felt structures determine the important properties of the membranes. Thus, in special reverse osmosis membranes for example, the polymer chains of the separation layer are on average so dense that, despite a good dissolving effect on individual water molecules, the formation of water clusters does not for the most part occur. It is presumably for the same reason that the separation layer is unable to take up any hydrated ions. This is the basis for its high retention power for dissolved salts.
Membrane separation processes have a high thermodynamic useful effect by comparison with separation processes which rely on phase transitions (for example evaporation, distillation). In addition, they do not cause any waste-heat problems. However, their use on a commercial scale is still hampered by a number of unsolved problems:
The throughput of most of the HFM's hitherto developed is too low. This is attributable inter alia to the excessive thickness of the separation layers. Constant separation layer thicknesses of less than 50 .ANG. are desirable. This would mean a reduction in thickness by a factor of 5 to 10 in relation to the separation layer thicknesses commonly used today. Although there have been several reports of separation layer thicknesses of the order of 50 .ANG., the thickness values in question are generally the minimal thicknesses of separation layers whose thickness is in any case subject to considerable variation. These variations are attributable to the fact that the separation layers consist entirely of polymers having a predominantly amorphous felt structure which fill out the unevennesses in the underlying surface of the supporting layers. Another reason for the excessively low throughput of HFM's lies in the fact that their porous structures undergo increasing compaction with increasing pressure differences.
The permeability and hence the throughput of the amorphous separation layers is additionally impaired by the fact that all the diffusion coefficients are unnecessarily reduced by indirect factors.
Aside from special cases, the separation power of hitherto developed HFM's is inadequate. The felt structure of the separation layers is again responsible for this. The significant local variations in the intervals between the polymer chains of a predominantly amorphous layer do not provide for high specificity. Not one of the processes used for applying the separation layers enables average intervals to be adjusted exactly between the polymer chains. Separation layers having substantially constant and exactly adjustable intervals between their chains are desirable.
The useful lives of hitherto developed membranes are too short. This is due primarily to so-called "fouling". Fouling is understood to be the adsorption of--mostly negatively charged--colloidal impurities and micro-organisms on the surfaces of the separation layers. This disadvantage is also due in part to the amorphous structure of the separation layers. Fouling is successfully prevented by the application of negative fixed charges to the outer surface of the separation layers. Under the effect of these fixed charges, the impurities referred to are electrostatically repelled. The repelling effect increases with increasing fixed charge density. However, the charge densities attainable with crosslinked or uncrosslinked amorphous linear polyelectrolytes are below the theoretically attainable maximum charge densities. Negative charge densities substantially corresponding to a very close packing of the fixed charges are desirable.
In a number of special cases, it has been possible to produce HFM's which show either a high throughput or a high power of separation or long useful lives. However, it has not been possible to combine all three required properties in one and the same membrane. The reason for this--as already explained--lies primarily in the amorphous structure of the separation layers.